1. Field of the Invention
The present invention relates to the field of fabrication of integrated circuits, and more particularly, to an apparatus for metrology by means of scatterometry.
2. Description of the Related Art
Fabrication of integrated circuits requires tiny regions of precisely controlled size to be formed in a material layer of an appropriate substrate, such as a silicon substrate. These tiny regions of precisely controlled size are generated by treating the material layer by means of, for example, ion implantation or etching, wherein a mask layer is formed over the material layer to be treated to define these tiny regions. In general, a mask layer may consist of a layer of photoresist that is patterned by a lithographic process. During the lithographic process, the resist may be spin coated onto the wafer substrate, and is then selectively exposed to ultraviolet radiation. After developing the photoresist, depending on the type of resist, positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the photoresist layer. Since the dimensions of the patterns in modem integrated circuits are steadily decreasing, the equipment used for patterning device features have to meet very stringent requirements with regard to resolution of the involved fabrication processes. In this respect, resolution is considered as a measure specifying the consistent ability to print minimum-size images under conditions of predefined manufacturing variations. One dominant factor in improving the resolution is represented by the lithographic process, in which patterns contained in a photo mask or reticle are optically transferred to the substrate via an optical imaging system. Therefore, great efforts are made to steadily improve optical properties of the lithographic system, such as numerical aperture, depth of focus, and wavelength of the light source used.
The quality of the lithographic imagery is extremely important in creating very small feature sizes. Of comparable importance is, however, the accuracy with which an image can be positioned on the surface of the substrate. Integrated circuits are fabricated by sequentially patterning material layers, wherein features on successive material layers bear a spatial relationship to one another. Each pattern formed in a subsequent material layer has to be aligned to a corresponding pattern formed in the previous material layer within specified registration tolerances. These registration tolerances are caused by, for example, a variation of a photoresist image on the substrate due to non-uniformities in such parameters as resist thickness, baking temperature, exposure and development. Furthermore, non-uniformities in the etching processes can lead to variations of the etched features. In addition, there exists an uncertainty in overlaying the image of the pattern for the current material layer to the etched pattern of the previous material layer, while photolithographically transferring the image onto the substrate. Several factors contribute to the inability of the imagery system to perfectly overlay two layers, such as imperfections within a set of masks, temperature differences between times of exposure, and a limited registration capability of the alignment tool. As a result, the dominant criteria determining the minimum feature size finally obtained are resolution for creating features in individual wafer levels and the total overlay error to which the above-explained factors, in particular the lithographic processes, contribute.
Accordingly, it is essential to steadily monitor the resolution, i.e. the capability of reliably and reproducibly creating the minimum feature size, also referred to as critical dimension (CD), within a specific material layer, and to steadily determine the overlay accuracy of patterns of two subsequently formed material layers. Recently, scatterometry has become a powerful tool in characterizing a periodic pattern of features with a size in the range of 1 xcexcm to 0.1 xcexcm. In the scatterometry analysis, the substrate containing a periodic structure is illuminated with radiation of an appropriate wavelength range and the diffracted light is detected. Many types of apparatus may be used for illumination and detecting of the diffracted light beam. U.S. Pat. No. 5,867,276 describes a so-called two-xcex8 scatterometer wherein the angle of incidence of a light beam is continuously varied by synchronously rotating the sample and the detector. Furthermore, this document describes a lens scatterometer system utilizing a rotating block to translate a light beam emitted from a light source to different points of the entrance aperture of a lens to illuminate the substrate at different angles of incidence. Moreover, this document describes a scatterometer with a fixed angle of incidence that utilizes a multi-wavelength illumination source to obtain the required information from the diffracted multi-wavelength beam. From this information contained in the measurement spectrum, the optical and dimensional properties of the individual elements that form the periodic structure and thickness of underlying films can be extracted, for example, by statistical techniques. The sample parameters of interest may include the width of lines, if the periodic pattern contains lines and spaces, their sidewall angle, and other structural details. In case of a more complex periodic structure having, for example, a two-dimensional periodicity, the parameters may include dimensional properties such as hole diameter or depth. It should be noted that in the present application the term xe2x80x9cscatterometerxe2x80x9d also includes devices emitting a substantially linearly polarized light beam such as an ellipsometer, to obtain structural information with respect to changes in the polarization state by detecting and analyzing the beam scattered from the periodic structure.
Typically, the diffracting patterns used for metrology of critical dimensions and overlay accuracy exhibit a periodicity along a predefined direction. Accordingly, two diffracting patterns are provided having a periodicity defined along two orthogonal directions to precisely monitor the quality of critical dimensions with respect to both directions. This arrangement, however, requires rotating the substrate and re-aligning the substrate with respect to the measurement tool, such as a spectroscopic ellipsometer, which is frequently used in semiconductor facilities. Rotating and re-aligning the substrate, however, significantly reduces the throughput.
Therefore, a need exists for an apparatus used for metrology of critical dimensions and overlay accuracy that allows precise measurements with high efficiency.
In view of the problems pointed out above, according to one aspect of the present invention an apparatus is provided for measuring critical dimensions and overlay errors in a semiconductor structure, wherein the semiconductor structure includes a first diffracting pattern oriented in a first direction and second diffracting pattern oriented in the second direction. The apparatus in accordance with the present invention comprises a light source for emitting at least one light beam, a first plurality of deflecting elements defining a first optical path to direct a first light beam to the first diffracting pattern and a second plurality of deflecting elements defining a second optical path to direct a second light beam to the second diffracting pattern. The apparatus further comprises a detector optically connectable to the first and second optical paths to receive a light beam diffracted by the first and second diffracting patterns, respectively.
According to a second aspect of the present invention, an apparatus is provided for obtaining information on critical dimensions and overlay accuracy of features formed in a semiconductor structure, wherein the semiconductor structure includes a first diffracting pattern oriented with respect to a first direction and a second diffracting pattern oriented with respect to a second direction. The apparatus comprises a light source for emitting at least one light beam and at least one optical fiber to define a first optical path and second optical path. In addition, the apparatus comprises a detector optically connectable to the first and second optical paths to receive a light beam diffracted by the first and the second diffracting patterns, respectively.
The apparatus in accordance with the present invention allows the detection of first and second light beams that are diffracted by the differently-oriented first and second diffracting patterns. Thus, a time consuming rotation of the substrate and a re-alignment is no longer necessary. The present invention is particularly advantageous in combination with existing scatterometers, such as a spectroscopic ellipsometer, since the ellipsometer may be used as both light source and detecting means of the apparatus in accordance with the present invention. Accordingly, to obtain the complete information on critical dimensions or to obtain the full overlay information in case the first and the second diffracting patterns bear information on the overlay accuracy of features formed by means of two subsequent photo-lithographic processes, one or more measurement sites, i.e., one or more diffracting patterns, can be visited in an uninterrupted sequence. The stage holding the wafer will linearly be moved for the distance between two diffracting patterns without rotation of the wafer. As a result, compared to a conventional apparatus, such as a spectroscopic ellipsometer, a significant increase of throughput is accomplished.